Diagnosis
The plug is connected to your local WiFi but has lost its connection to the Kasa cloud servers. The app relies on the cloud for remote control.
How to Fix It
Restart your home router. This refreshes all network connections. After the router is back online, check the app again. The plug should reconnect to the cloud within a minute or two.
If That Didn't Work
There might be a temporary TP-Link cloud outage. Check TP-Link's service status page or social media. If the cloud is up, try force-closing and reopening the Kasa app, or logging out and back into the app.
Heads Up
A solid green light means local WiFi is good, so you can still control the plug manually with the side button. Cloud dependency is a limitation of these budget smart plugs—if your internet or their servers are down, remote app control won't work.